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| United States Patent |
5,695,700
|
|
Takeuchi
, et al.
|
December 9, 1997
|
Method of preparing a ceramic porous body
Abstract
A ceramics porous body having high porosity as well as high strength is
especially suitable for use as a filter for removing foreign matter from a
fluid or as a catalytic carrier. The porous body has a porosity of at
least 30% and comprises columnar ceramic grains having an aspect ratio of
at least 3. In particular, the porous body comprises Si.sub.3 N.sub.4
grains, of which at least 60% are hexagonal columnar .beta.-Si.sub.3
N.sub.4 grains. The porous body further comprises at least one compound of
a rare earth element in an amount of at least 1 volume % and not more than
20 volume % in terms of an oxide of the rare earth element, and optionally
at least one compound of elements of the groups IIa and IIIb of the
periodic table and transition metal elements in an amount of not more than
5 volume % in terms of an oxide of each element. A compact of mixed powder
obtained by adding the compound powder of the rare earth element to
silicon nitride powder is heat treated in a nitrogen atmosphere at a
temperature of at least 1500.degree. C., to prepare the silicon nitride
ceramic porous body.
| Inventors:
|
Takeuchi; Hisao (Itami, JP);
Nakahata; Seiji (Itami, JP);
Matsuura; Takahiro (Itami, JP);
Kawai; Chihiro (Itami, JP)
|
| Assignee:
|
Sumitomo Electric Industries, Ltd. (Osaka, JP)
|
| Appl. No.:
|
450379 |
| Filed:
|
May 25, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
264/626; 264/42; 264/43; 264/628 |
| Intern'l Class: |
C04B 033/34; C04B 033/36; C04B 035/71; C04B 037/00 |
| Field of Search: |
264/60,42,43,44
|
References Cited
U.S. Patent Documents
| 4332909 | Jun., 1982 | Nishida et al.
| |
| 4629707 | Dec., 1986 | Wolfe.
| |
| 5269989 | Dec., 1993 | Pyzik et al.
| |
| Foreign Patent Documents |
| 0123292 | Oct., 1984 | EP.
| |
| 3835807 | May., 1989 | DE.
| |
| 56-75546 | Jun., 1981 | JP.
| |
| 61-53176 | Mar., 1986 | JP.
| |
| 62-18621 | Jan., 1987 | JP.
| |
| 63-156070 | Jun., 1988 | JP.
| |
| 63-291882 | Nov., 1988 | JP.
| |
| 1-93469 | Apr., 1989 | JP.
| |
| 1-188479 | Jul., 1989 | JP.
| |
| 2-089812 | Mar., 1990 | JP.
| |
| 3150275 | Jun., 1991 | JP.
| |
| 3-170376 | Jul., 1991 | JP.
| |
| 3-281740 | Dec., 1991 | JP.
| |
| 4-37668 | Feb., 1992 | JP.
| |
| 4-219374 | Aug., 1992 | JP.
| |
| 4-285079 | Oct., 1992 | JP.
| |
| 4-357170 | Dec., 1992 | JP.
| |
| 6116054 | Apr., 1994 | JP.
| |
Other References
JIS R 1601 (English version).
Journal of Ceramic Society of Japan 100(5) 758-762 (1992) Microstructure
and Electrical-Properties in a Humid Atmosphere by Susumu Nakayama et al.
Ceramic Transactions, ISSN 1042-1122, vol. 31, Porous Materials (1992).
|
Primary Examiner: Czaja; Donald E.
Assistant Examiner: Ruller; Jacqueline A.
Attorney, Agent or Firm: Fasse; W. G., Fasse; W. F.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a Divisional of U.S. patent application Ser. No. 08/367,220, filed
Jan. 6, 1995 now U.S. Pat. No. 5,618,765 which in turn is the national
stage filing of PCT/JP 94/00803 filed May 19, 1994.
Claims
We claim:
1. A method of producing a ceramic porous body comprising the steps:
(a) preparing a mixed powder by mixing together a silicon nitride powder
and at least one compound powder of a rare earth element in an amount of
at least 1 volume % and not more than 20 volume % in terms of an oxide of
said rare earth element;
(b) pressing said mixed powder to form a compact; and
(c) heat treating said compact in a nitrogen atmosphere at a temperature of
at least 1700.degree. C. and not more than 2100.degree. C.;
so as to produce said ceramic porous body to contain silicon nitride grains
including columnar .beta. silicon nitride grains having a hexagonal
cross-section in a content ratio of at least 60% of said columnar .beta.
silicon nitride grains relative to a total of all of said silicon nitride
grains, to have a porosity of at least 30%, and to have an aspect ratio of
said columnar .beta. silicon nitride grains in the range of at least 3 and
not more than 20.
2. The method of claim 1, wherein said silicon nitride powder used in said
step (a) has a mean grain size in the range of at least 0.1 .mu.m and not
more than 20 .mu.m, and said step (b) comprises controlling a density of
said compact to be in a range of at least 30% and not more than 60%.
3. The method of claim 1, wherein said silicon nitride powder used in said
step (a) contains mainly .beta. silicon nitride.
4. The method of claim 1, wherein said silicon nitride powder used in said
step (a) is .beta. silicon nitride powder.
5. The method of claim 1, wherein said silicon nitride powder used in said
step (a) contains mainly amorphous silicon nitride.
6. The method of claim 1, wherein said silicon nitride powder used in said
step (a) is amorphous silicon nitride powder.
7. The method of claim 1, wherein said compound powder of a rare earth
element used in said step (a) comprises at least one compound selected
from the group consisting of alkoxides, hydroxides and nitrides of said
rare earth element.
8. The method of claim 1, carried out so that said ceramic porous body has
pores with a mean pore size of at least 0.05 .mu.m and not more than 12
.mu.m.
9. The method of claim 8, wherein said mean pore size is not more than 4
.mu.m and said porosity is greater than 50%.
10. The method of claim 8, wherein said porosity is at least 40%, and a
bending strength of said porous body is at least 100 MPa.
11. A method of producing a ceramic porous body comprising the steps:
(a) preparing a mixed powder by mixing together a silicon nitride powder,
at least one first compound powder of a rare earth element in an amount of
at least 1 volume % and not more than 20 volume % in terms of an oxide of
said rare earth element, and at least one second compound powder of an
element selected from the group IIa, the group IIIb and the transition
metal elements of the periodic table in a selected volume percentage
expressed in terms of an oxide of said element and selected from the group
consisting of a first percentage of more than 0 volume % and not more than
1 volume %, a second percentage of at least 1 volume % and not more than 2
volume %, and a third percentage of more than 2 volume % and not more than
5 volume %;
(b) pressing said mixed powder to form a compact; and
(c) heat treating said compact in a nitrogen atmosphere at a selected
temperature in a first temperature range of at least 1600.degree. C. and
not more than 1900.degree. C. when said mixed powder prepared in said step
(a) contains said first percentage of said second compound powder, in a
second temperature range of at least 1600.degree. C. and not more than
1850.degree. C. when said mixed powder prepared in said step (a) contains
said second percentage of said second compound powder, and in a third
temperature range of at least 1500.degree. C. and not more than
1700.degree. C. when said mixed powder prepared in said step (a) contains
said third percentage of said second compound powder;
so as to produce said ceramic porous body to contain silicon nitride grains
including columnar .beta. silicon nitride grains having a hexagonal
cross-section in a content ratio of at least 60% of said columnar .beta.
silicon nitride grains relative to a total of all of said silicon nitride
grains, to have a porosity of at least 30%, and to have an aspect ratio of
said columnar .beta. silicon nitride grains in the range of at least 3 and
not more than 20.
12. The method of claim 11, wherein said mixed powder prepared in said step
(a) contains said first percentage of said second compound powder, and
said selected temperature for said heat treating of said step (c) is in
said first temperature range.
13. The method of claim 11, wherein said mixed powder prepared in said step
(a) contains said second percentage of said second compound powder, and
said selected temperature for said heat treating of said step (c) is in
said second temperature range.
14. The method of claim 11, wherein said mixed powder prepared in said step
(a) contains said third percentage of said second compound powder, and
said selected temperature for said heat treating of said step (c) is in
said third temperature range.
15. The method of claim 11, wherein said silicon nitride powder used in
said step (a) is .beta. silicon nitride powder.
16. The method of claim 11, wherein said silicon nitride powder used in
said step (a) is amorphous silicon nitride powder.
17. The method of claim 11, wherein said compound powder of a rare earth
element used in said step (a) comprises at least one compound selected
from the group consisting of alkoxides, hydroxides and nitrides of said
rare earth element.
18. The method of claim 11, carried out so that said ceramic porous body
has pores with a mean pore size of at least 0.05 .mu.m and not more than
12 .mu.m.
19. The method of claim 18, wherein said mean pore size is not more than 4
.mu.m and said porosity is greater than 50%.
20. The method of claim 18, wherein said porosity is at least 40%, and a
bending strength of said porous body is at least 100 MPa.
Description
FIELD OF THE INVENTION
The present invention generally relates to a ceramics porous body which is
useful as a filter material for removing foreign matter from a fluid or as
a catalytic carrier, and more specifically, it relates to a silicon
nitride ceramics porous body and a method of preparing the same.
BACKGROUND ART
Known porous bodies employed for filter materials or catalytic carriers,
include those consisting of various materials such as resin, metals or
ceramics. Among these, a filter or a catalytic carrier consisting of a
ceramics material is generally employed in high temperature or strongly
corrosive environments which cannot with stand other materials. A filter
or a catalytic carrier consisting of oxide ceramics such as alumina
(Al.sub.2 O.sub.3) has already been put into practice.
As to a porous body consisting of nonoxide ceramics, on the other hand,
only small examples have been put into practice while Japanese Patent
Laying-Open No. 63-291882 discloses a silicon nitride based or silicon
carbide based porous body prepared by a heat treatment. Further, Japanese
Patent Laying-Open No. 1-188479 discloses a method of compacting mixed
powder of silicon powder and silicon nitride powder of relatively coarse
particles and thereafter nitriding the same thereby preparing a porous
body as a solid target.
As hereinabove described, it is difficult to use a porous body consisting
of resin or a metal in a high temperature or corrosive atmosphere. It is
inevitably necessary to employ a porous body made of ceramics for a filter
for removing foreign matter from a high-temperature exhaust gas or for a
carrier serving as a catalyst for decomposing a harmful matter.
As an example of such porous bodies made of ceramics, porous bodies made of
alumina have been put into practice. While the porous bodies of alumina
are varied in pore size, porosity and bending strength, a porous body
having a porosity of 35 to 40% and a mean pore size of 25 to 130 .mu.m has
a bending strength of 20 to 35 MPa, whereby the strength of the porous
body is insufficient depending on its use.
In the silicon nitride based porous body disclosed in the aforementioned
Japanese Patent Laying-Open No. 63-291882, the porosity is less than 30%
and fluid permeability is insufficient. In general, the strength of
ceramics tends to be reduced following an increase in the porosity, and it
has been extremely difficult to attain compatibility between the porosity
and the strength.
SUMMARY OF THE INVENTION
In view of the above, the present invention has been proposed in order to
solve the aforementioned problems, and an object thereof is to provide a
ceramics porous body having high porosity as well as high strength.
The present inventors have deeply studied the aforementioned subject, and
discovered that it is possible to prepare a silicon nitride ceramics
porous body that is mainly composed of columnar .beta.-Si.sub.3 N.sub.4
(.beta.-silicon nitride) crystal grains and capable of maintaining high
strength also when its porosity is high, by heat treating a compact of
mixed powder of silicon nitride (Si.sub.3 N.sub.4) powder and prescribed
additive powder at a high temperature.
Namely, a ceramic porous body according to the present invention is
generally characterized in that it has a porosity of at least 30% and is
mainly composed of columnar ceramics grains having an aspect ratio of at
least 3. Specifically, it is a porous body having a mean pore size of at
least 0.05 .mu.m and not more than 12 .mu.m. Further, the crystal grains
preferably have the shape of hexagonal poles or columns, i.e. rod-like
crystal shape with a hexagonal cross-section
More specifically, the ceramics porous body according to the present
invention is a silicon nitride ceramics porous body that is mainly
composed of silicon nitride with a ratio of at least 60%, preferably at
least 90%, of .beta.-Si.sub.3 N.sub.4 columnar grains with respect to the
entire silicon nitride grains, contains at least one compound of a rare
earth element with at least 1 volume % and not more than 20 volume % of an
oxide of the rare earth element, and has a porosity of at least 30%.
The aforementioned silicon nitride ceramics porous body may contain at
least one compound of elements of the groups IIa and IIIb of the periodic
table and transition metal elements, with not more than 5 volume % of an
oxide of each element. Further, the silicon nitride ceramics porous body
according to the present invention preferably has bending strength of at
least 80 MPa at an ordinary temperature i.e. room temperature, and bending
strength of at least 50 MPa at a temperature of 1000.degree. C.
In summary, further, a method of preparing a silicon nitride ceramics
porous body according to the present invention comprises the following
steps.
A first step involves adding at least one compound powder of a rare earth
element in an amount corresponding to at least 1 volume % and not more
than 20 volume % of an oxide of the rare earth element, or further adding
at least one compound of elements of the groups IIa and IIIb of the
periodic table and transition metal elements in an amount corresponding to
not more than 5 volume % of an oxide of each element, to silicon nitride
powder, thereby preparing a mixed powder.
A subsequent step involves preparing a compact from the aforementioned
mixed powder.
A further subsequent step involves heat treating the compact in a nitrogen
atmosphere at a temperature of at least 1500.degree. C. and not more than
2100.degree. C.
In the present invention, the compound of a rare earth element acts to
react with SiO.sub.2 existing on the surface of the raw material of the
silicon nitride (Si.sub.3 N.sub.4) powder during the heat treatment for
forming a liquid phase and solidly dissolving Si.sub.3 N.sub.4, thereby
precipitating columnar .beta.-Si.sub.3 N.sub.4 crystal grains. Further,
the compound of the rare earth element acts to exist outside the
.beta.-Si.sub.3 N.sub.4 grains as a grain boundary phase after the heat
treatment, for joining the .beta.-Si.sub.3 N.sub.4 grains and maintaining
strength. The rare earth element indicates an Sc, Y or lanthanoid element.
The ratio of the added compound of the rare earth element is suitably in
the range of 1 to 20 volume % of an oxide of the rare earth element, and
more preferably 2 to 10 volume %. The form of the grain boundary phase is
a silicate such as Y.sub.2 O.sub.3.SiO.sub.2, or an oxynitride such as
Y.sub.2 O.sub.3.Si.sub.3 N.sub.4. Columnarization of the .beta.-Si.sub.3
N.sub.4 crystal grains is not sufficient if the added quantity of the
compound of the rare earth element is less than 1 volume %, while
oxidation resistance and strength at a high temperature are reduced if the
amount exceeds 20 volume %, which further leads to increase in preparation
cost since the rare earth element is generally high-priced.
The compound(s) of the element(s) of the group(s) IIa and/or IIIb of the
periodic table and/or the transition metal element(s) is/are added when a
sintered body is prepared, in general. The aforementioned compound of the
rare earth element acts to reduce a liquid phase forming temperature,
facilitate densification and improve strength when the same is employed
with the compound(s) of the element(s) of the group(s) IIa and/or IIIb of
the periodic table and/or the transition metal element(s). The elements of
the group IIa of the periodic table are Be, Mg, Ca, Sr and the like, the
elements of the group IIIb are B, Al, Ga and the like, and the transition
metal elements are Fe, Ti, Zr and the like.
In order to prepare a porous body having high porosity, the added or
additional ratio of the compound of such an element is preferably small.
The added amount is suitably not more than 5 volume % of an oxide of each
element, preferably not more than 2 volume %, and more preferably not more
than 1 volume %.
Due to addition of the compound(s) of the element(s) of the group(s) IIa
and/or IIIb of the periodic table and/or the transition metal element(s),
on the other hand, the liquid phase is formed in a lower temperature
region, whereby grain growth also takes place in a low temperature region.
This is conceivably because the grain growth is caused by re-precipitation
of Si.sub.3 N.sub.4 which is dissolved in the liquid phase, to reduce a
grain growth starting temperature. When the compound(s) of the element(s)
of the group(s) IIa and/or IIIb and/or the transition metal element(s)
is/are added, therefore, it is possible to obtain a high-strength porous
body at a low temperature, thereby attaining an advantage in view of the
preparation cost. Further, such grain growth in a low temperature region
tends to form fine crystal grains, whereby it is possible to prepare a
porous body having a small pore size.
When the additional ratio of the compound(s) of the element(s) of the
group(s) IIa and/or IIIb and/or the transition metal element(s) exceeds 5
volume %, densification is disadvantageously caused before columnar grain
growth takes place to reduce porosity of the porous body while oxidation
resistance is reduced due to a high densification effect from the low
temperature region.
Particularly when a compound of a IVa group element such as Ti among the
transition metals is added, the compound reacts with .beta.-Si.sub.3
N.sub.4 at a high temperature of at least 1600.degree. C. and it is
possible to increase the bonding strength between the crystal grains,
whereby a porous body of high strength can be obtained.
While the Si.sub.3 N.sub.4 powder employed as a raw material is mainly
composed of .alpha.-Si.sub.3 N.sub.4 in general, .beta.-Si.sub.3 N.sub.4
or amorphous silicon nitride may alternatively be employed as the raw
material. A mean grain size of the silicon nitride powder is preferably at
least 0.1 .mu.m and not more than 20 .mu.m. If the mean grain size of the
silicon nitride powder is less than 0.1 .mu.m, agglomeration of the powder
materials is so intensely caused that the relative density of the compact
as obtained is not more than 30%, and the handling strength of the compact
as well as the strength of the porous body after the heat treatment are
insufficient. When the mean grain size of the silicon nitride powder
exceeds 20 .mu.m, on the other hand, the degree of sintering by the heat
treatment is reduced and the porous body cannot attain a strength of at
least 80 MPa.
Most generally the aforementioned compound of the rare earth element and
the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the
periodic table and/or the transition metal element(s) is/are added as
oxide powder materials, but it is also possible to add the same as
compounds such as hydroxides or alkoxides which are decomposed to form
powder materials of hydroxides or oxides. It is also possible to add these
compounds in the form of nitride powder materials or the like.
These powder materials are mixed with each other by a prescribed method
such as a ball mill method, and thereafter compacted. Also as to the
compacting method, it is possible to employ a prescribed method such as
die pressing or CIP (cold isostatic pressing). The compact density varies
with the characteristics of the powder materials and the target porosity
of the porous body.
In order to facilitate growth of columnar grains as well as to attain a
high porosity, the compact density is preferably low. In order to ensure
attainment of the strength that is required for handling the compact, and
to improve the strength of the porous body after the heat treatment,
however, it is necessary to prepare the compact with a compact density
exceeding a certain constant level. When commercially available
.alpha.-Si.sub.3 N.sub.4 powder is employed, it is preferable to set the
compact density at 30 to 60% of theoretical density, more preferably at 35
to 50%. If only the compound of the rare earth element is added, the
porosity after the heat treatment exceeds 30% when the compact density is
less than 30% in relative density, while the pore size is also increased
and a porous body having high bending strength cannot be obtained even if
columnar crystals are formed. When the compact density exceeds 60% in
relative density, on the other hand, it is possible to attain sufficiently
high bending strength in the porous body, while porosity is less than 30%
and the pore size is also reduced.
The compact as obtained is heat treated in a nitrogen atmosphere at a
temperature of at least 1500.degree. C. after a compacting assistant
(resin or the like) is removed by thermal decomposition or the like.
Transition to .beta.-Si.sub.3 N.sub.4 (in a case of employing .alpha.
powder) and grain growth (columnarization) proceed by the heat treatment,
so that the compact is converted to a porous body mainly consisting of
.beta.-Si.sub.3 N.sub.4 columnar grains. The heat treatment temperature
varies with the composition of the additive, the grain size of the raw
material powder, and the mean pore size and the porosity of the target
porous body.
When only a compound of a rare earth element such as Y.sub.2 O.sub.3 is
added, for example, it is necessary to carry out the heat treatment in a
high temperature region of at least 1700.degree. C. In this case, no
remarkable densification proceeds even if the heat treatment is carried
out at a higher temperature, and hence it is also possible to carry out
the heat treatment in a temperature region extremely increasing the pore
size. When the compound(s) of the element(s) of the group(s) IIa and/or
IIIb of the periodic table and/or the transition metal element(s) is/are
added in addition to the compound of the rare earth element, on the other
hand, a liquid phase is formed in a low temperature region and Si.sub.3
N.sub.4 which is dissolved in this liquid phase is precipitated as
columnar .beta.-grains as described above, whereby it is possible to
prepare a porous body of high strength also by a heat treatment in a low
temperature region. However, a heat treatment which is carried out at a
high temperature is improper as a method of preparing a porous body due to
progress of densification. The densification is readily facilitated and
the porosity is readily reduced as the additional amount of the
compound(s) of the element(s) of the group(s) IIa and/or IIIb and/or the
transition metal element(s) is increased.
Therefore heat treatment temperature for the compact is preferably in the
range of 1600.degree. to 1900.degree. C. amount of addition of the
compound(s) of the element(s) of the group(s) IIa and/or IIIb and/or the
transition metal element(s) is in excess of 0 volume % and not more than 1
volume %, 1600.degree. to 1850.degree. C. if the amount of addition of the
compound is in excess of 1 volume % and not more than 2 volume %, and 1500
to 1700.degree. C. if the amount of addition is in excess of 2 volume %
and not more than 5 volume %. In general, grain growth is not sufficient
if the heat treatment temperature for the compact is less than
1500.degree. C.
Since silicon nitride is increased in decomposition pressure at a high
temperature, it is necessary to increase a nitrogen partial pressure with
the heat treatment temperature. The atmosphere of the heat treatment may
be an inert atmosphere containing nitrogen, and a mixed atmosphere of
argon (Ar) or the like may be employed. A temperature of at least
1700.degree. C. is required when no compound(s) of the element(s) of the
group(s) IIa and/or IIIb of the periodic table is added, but a heat
treatment at a temperature exceeding 2100.degree. C. is advantageous for
preparation of a porous body having a large pore size due to extreme grain
growth. However, the nitrogen partial pressure must be at least several
100 atm. in order to control the porosity and the bending strength to be
in the inventive ranges, i.e. at least 30% and at least 80 MPa at room
temperature and at least 50 MPa at a temperature of 1000.degree. C.
respectively, and the cost is disadvantageously increased in view of the
apparatus required to achieve this. When the heat treatment is carried out
at a temperature exceeding 2100.degree. C., further, the use of the porous
body is also disadvantageously restricted due to a tendency that the
bending strength of the porous body is also reduced. Thus, the heat
treatment temperature is preferably not more than 2100.degree. C.
The porous body thus obtained has a structure in which .beta.-Si.sub.3
N.sub.4 columnar crystal grains are joined with each other by a grain
boundary phase that is formed from the compound of the rare earth element,
the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the
periodic table and/or the transition metal element(s), or a Si substance
derived from the Si.sub.3 N.sub.4 powder, and that exhibits high strength
also when the porosity is high. It is conceivable that the reasons why
such a high strength is achieved are that the inventive porous body has a
structure in which the columnar crystal grains are entangled with each
other dissimilarly to a generally employed Al.sub.2 O.sub.3 porous body
having a polycrystalline network structure consisting of spherical crystal
grains, and that the columnar grains have extremely high strength (several
GPa) since they are single crystals having substantially no defects.
In this porous body, it is possible to arbitrarily control the mean pore
size within the range of at least 0.05 .mu.m and not more than 12 .mu.m by
the grain size of the raw material powder and the compact density. If the
mean pore size is less than 0.05 .mu.m, development of the columnar grains
is not sufficient and the aspect ratio thereof is less than 3. As a
result, the porosity is disadvantageously reduced. When the mean pore size
exceeds 12 .mu.m, on the other hand, the sizes of the crystal grains
exceed 36 .mu.m in length and 12 .mu.m in breadth, which reduces the
strength. Therefore, it is possible to employ the inventive porous body in
the field of microfiltration etc. in a higher temperature environment or
in an arrangement in which it receives a load, by controlling the mean
pore size within the aforementioned range.
The ratio of .beta.-Si.sub.3 N.sub.4 forming the columnar grains is
preferably at least 60% of the entire Si.sub.3 N.sub.4, and more
preferably at least 90% thereof. The ratio of .beta.-Si.sub.3 N.sub.4, is
thus defined at an extremely high value, since .alpha.-Si.sub.3 N.sub.4,
which is another crystal form of Si.sub.3 N.sub.4, exhibits a spherical
shape and causes a reduction in strength. When .beta.-silicon nitride
columnar grains are at least 60% and less than 90% of the entire silicon
nitride grains, the crystal structure thereof is in such a form that
.alpha.-silicon nitride grains and .beta.-silicon nitride columnar grains
are composed with each other. In this case, the .beta.-columnar grains
couple portions where .alpha.-crystal grains exist with each other,
whereby it is possible to attain higher strength than that having a
.beta.-transition ratio of less than 60%. Further, growth of such columnar
grains also serves to prevent densification. Since Si.sub.3 N.sub.4
exhibits high oxidation resistance, the silicon nitride ceramics porous
body can be employed without breakage even when a high load is applied at
a high temperature. Further, the silicon nitride ceramics porous body
according to the present invention has high strength and a low coefficient
of thermal expansion, whereby it also has excellent characteristics as to
a thermal shock.
While specially a silicon nitride ceramics porous body has been described
above, the inventive body is not limited thereto because both high
porosity and high strength essentially result from a structure in which,
the columnar grains are entangled with each other, regardless of the
specific ceramic composition. Thus, another material having such a
structure in which columnar grains are entangled with each other also
provides a similar effect. For example, such behavior is recognized also
in aluminum nitride containing Si and a sintering assistant such as an
oxide of a rare earth element as impurities. In general, therefore, it is
possible to attain the aforementioned effect in a ceramics porous body
having a porosity of at least 30% when the same is mainly composed of
columnar ceramics grains having an aspect ratio of at least 3. The porous
body is excellent in the aforementioned effect when the aspect ratio,
which indicates a ratio of the length to the breadth of the columnar
grains, is high in general, while the effect of improvement in strength is
small if the aspect ratio is less than 3. In many samples according to the
invention, the aspect ratio is at least 10. The highest aspect ratio for
any of the inventive samples is 20 (see e.g. the eighth and ninth samples
in Table 4.1).
Further, the columnar grains of the silicon nitride ceramics porous body
have a hexagonal pole or rod shape having a hexagonal cross-section. In
this case, the pores are formed by side surfaces of the hexagonal poles.
It has been proved as the result of study by the inventors that, when the
side surfaces, which are planes, are covered with a metal (platinum, for
example) serving as a catalyst, the metal can uniformly adhere onto the
surfaces and is thereby to be improved in performance as a catalyst.
DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE
OF THE INVENTION
EXAMPLE 1
An yttrium oxide powder material of 0.5 .mu.m mean grain size (specific
surface area: 7 m.sup.2 /g) was added to a silicon nitride powder material
mainly composed of .alpha.-silicon nitride (.alpha.-Si.sub.3 N.sub.4) of
0.3 .mu.m mean grain size (specific surface area: 11 m.sup.2 /g), and
mixed with ethanol as a solvent in a ball mill for 72 hours. Amounts of
addition of the yttrium oxide powder material are shown in Table 1.
Mixed powder materials obtained in the aforementioned manner were dried and
thereafter compacted using a metal die of 100 mm.times.100 mm dimensions
under a pressure of 20 kg/cm.sup.2 with addition of a compacting
assistant. Compacts as obtained were about 15 mm in thickness and about
35% in relative density in every composition. The relative density was
determined by dividing the compact density, which was calculated from
measurements of the weight and dimensions, by the theoretical density,
which was a weighted mean of silicon nitride and the additive.
The compacts as obtained were heat treated under conditions shown in Table
1, thereby obtaining porous bodies. Test pieces of 3 mm.times.4
mm.times.40 mm in size for a three-point bending test in accordance with
JIS 1601 were cut out from the porous bodies. The test pieces were
employed for measuring bending strength values at an ordinary room
temperature and at 1000.degree. C. Further, porosity values were
calculated from the relative density values (porosity (%)=100 - relative
density (%)). In addition, .beta.-transition ratios were obtained from
X-ray diffraction peak intensity ratios, by carrying out X-ray diffraction
through the porous bodies as obtained. The calculation expression is shown
below.
(.beta.-transition ratio) (%)={A/(A+B)}.times.100
where A represents X-ray diffraction peak intensity of .beta.-silicon
nitride, and B represents an X-ray diffraction peak intensity ratio of
.alpha.-silicon nitride.
A scanning electron microscope (SEM) was employed to observe broken-out
sections, thereby obtaining mean crystal grain sizes. Mean pore sizes were
measured with a mercury porosimeter. These measurement results are shown
in Table 1.
TABLE 1
__________________________________________________________________________
Heat Treatment Conditions
Porous Body Characteristics
Additive Retention
Pressure of
Mean Crystal Grain Size
Bending Strength
.beta.-Transition
3
Y.sub.2 O.sub.3
Temperature
Time Atmosphere
Porosity
Pore Size
Length
Breadth
Room Temperature
1000.degree. C.
Ratio
No.
(Vol %)
(.degree.C.)
(H) (atm) (%) (.mu.m)
(.mu.m)
(.mu.m)
(MPa) (MPa)
(%)
__________________________________________________________________________
1 0 1800 2 4 60 0.5 -- 0.5 7 1 30
2 0.5
1800 2 4 45 0.8 1 0.5 80 80 100
3 1 1800 2 4 39 1.5 3 0.8 150 150 100
4 2 1800 2 4 48 1.8 12 0.8 130 100 100
5 4 1800 2 4 48 0.8 15 1.0 120 100 100
6 8 1800 2 4 58 3.5 20 1.5 100 85 100
7 12 1800 2 4 57 3.0 20 1.6 110 70 100
8 20 1800 2 4 55 4.0 18 1.8 100 60 100
9 30 1800 2 4 50 3.0 25 2.0 90 40 100
10 4 1500 2 4 61 0.3 -- 0.4 5 0.7 15
11 4 1600 2 4 60 0.4 1.5 0.4 6 0.8 20
12 4 1700 2 4 58 1.0 3 0.5 85 55 70
13 4 1700 2 4 56 2.0 10 0.8 100 80 90
14 4 1800 2 4 55 2.5 15 1.2 120 100 100
15 4 1900 2 10 55 3.5 20 1.5 110 100 100
16 4 2000 2 40 54 8.0 35 2.0 90 80 100
17 4 2100 2 100 54 12.0 50 3.0 80 60 100
18 4 1800 1 4 54 2.5 12 1.2 120 90 100
19 4 1800 5 4 55 3.5 20 1.5 110 90 100
20 4 1800 2 10 57 3.0 20 1.5 110 100 100
21 4 1650 2 4 53 0.8 2.0 0.6 61 38 50
22 4 1700 2 4 52 1.0 2.3 0.7 80 50 60
23 4 2100 10 100 25 13.0 42 3.8 40 32 100
24 2 1700 20 10 28 0.04
0.11 0.04
65 28 75
__________________________________________________________________________
EXAMPLE 2
Porous bodies were prepared by a method similar to that in Example 1 except
that oxide powder materials of respective rare earth elements shown in
Table 2 were employed as compounds of rare earth elements in place of
yttrium oxide powder materials, and evaluated. The results are shown in
Table 2. It is understood from the results that similar silicon nitride
porous bodies are obtained also when rare earth oxides other than yttrium
oxide are employed.
TABLE 2
__________________________________________________________________________
Porous Body Characteristics
Additive Heat Treatment Condition Bending Strength
Additional Retention
Pressure of Crystal Grain Size
Room .beta.-Transition
1
A Group
Ratio
Temperature
Time Atmosphere
Porosity
Pore Size
Length
Breadth
Temperature
1000.degree. C.
Ratio
Additive
(Vol %)
(.degree.C.)
(H) (atm) (%) (.mu.m)
(.mu.m)
(.mu.m)
(MPa) (MPa)
(%)
__________________________________________________________________________
La.sub.2 O.sub.3
4 1800 2 4 50 2.0 18 1.4 130 100 100
CeO.sub.2
4 1800 2 4 52 2.2 20 1.4 100 80 100
Nd.sub.2 O.sub.3
4 1800 2 4 48 2.2 15 1.4 130 90 100
Gd.sub.2 O.sub.3
4 1800 2 4 52 2.4 15 1.1 120 80 100
Dy.sub.2 O.sub.3
4 1800 2 4 53 2.5 16 1.3 110 90 100
Yb.sub.2 O.sub.3
4 1800 2 4 55 2.8 20 1.5 100 80 100
Y.sub.2 O.sub.3
4 1800 2 4 55 2.5 15 1.2 120 100 100
__________________________________________________________________________
EXAMPLE 3
Porous bodies were prepared by a method similar to that in Example 1 except
that yttrium oxide, being an oxide of a rare earth element, was added as
an A group additive, and aluminum oxide, magnesium oxide and titanium
oxide, being compounds of elements of the groups IIa and IIIb of the
periodic table and a transition metal element, were added as B group
additional compounds, and evaluated. The results are shown in Table 3.
As can be seen From Table 3 (which comprises Tables 3.1, 3.2, 3.3, and
3.4), it is understood that it is possible to prepare silicon nitride
porous bodies at lower temperatures in the present Example than in
Examples in which only rare earth oxides were added.
TABLE 3
__________________________________________________________________________
Porous Body Characteristics
Additive Heat Treatment Condition Bending
.beta.-h
A Addi-
B Addi- Pressure Crystal Room Trans-
Group tional
Group
tional
Temper-
Retention
of Atmo- Pore
Grain Size
Temper- ition
Addi-
Ratio
Addi-
Ratio
ature
Time sphere
Porosity
Size
Length
Breadth
ature
1000.degree.
Ratio
No.
tive
(Vol %)
tive
(Vol %)
(.degree.C.)
(H) (atm)
(%) (.mu.m)
(.mu.m)
(.mu.m)
(MPa)
(MPa)
(%)
__________________________________________________________________________
1 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0 1800 2 4 55 2.5
15 1.2 120 100 100
2 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0.5 1800 2 4 45 2 15 1.5 150 100 100
3 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
1.2 1800 2 4 28 1.9
15 1.5 170 120 100
4 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
2 1800 2 4 12 1.5
15 1.5 220 150 100
5 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
5 1800 2 4 2 1 12 1.5 540 350 100
6 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
10 1800 2 4 4 1 10 2 350 210 100
7 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0.5 1500 2 1 58 0.5
1.5 0.5 50 40 40
8 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0.5 1600 2 1 54 1.5
7 0.7 80 40 90
9 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0.5 1700 2 4 48 1.8
12 1 120 100 100
10 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0.5 1750 2 4 44 2.2
15 1.2 130 100 100
11 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
0.5 1900 2 10 40 2.5
20 2.2 130 110 100
12 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
2 1700 2 4 35 1 10 1.2 110 80 100
13 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
2 1750 2 4 31 1.3
15 1.3 140 80 100
14 Y.sub.2 O.sub.3
4 Al.sub.2 O.sub.3
5 1700 2 4 20 0.8
10 1.2 160 120 100
15 Y.sub.2 O.sub.3
4 MgO 0.5 1500 2 4 53 0.9
2 0.5 70 50 70
16 Y.sub.2 O.sub.3
4 MgO 1.2 1500 2 4 50 1 2.5 0.6 90 60 80
17 Y.sub.2 O.sub.3
4 MgO 2 1500 2 4 42 1 3 0.7 100 60 90
18 Y.sub.2 O.sub.3
4 MgO 5 1500 2 4 32 0.9
3 0.6 100 50 95
19 Y.sub.2 O.sub.3
4 MgO 10 1500 2 4 26 0.8
4 0.8 130 40 100
20 Y.sub.2 O.sub.3
4 MgO 0.5 1600 2 4 50 1.2
10 1.2 100 60 90
21 Y.sub.2 O.sub.3
4 MgO 1.2 1600 2 4 42 1.2
10 1 110 70 95
22 Y.sub.2 O.sub.3
4 MgO 2 1600 2 4 38 1.2
12 1 120 70 100
23 Y.sub.2 O.sub.3
4 MgO 5 1600 2 4 30 1 12 1.3 150 70 100
24 Y.sub.2 O.sub.3
4 MgO 10 1600 2 4 15 0.9
15 1.5 200 50 100
25 Y.sub.2 O.sub.3
4 MgO 0.5 1800 2 4 42 1.8
20 1.8 140 100 100
26 Y.sub.2 O.sub.3
4 MgO 1.2 1800 2 4 20 1.2
22 2 210 150 100
27 Y.sub.2 O.sub.3
4 MgO 2 1800 2 4 2 -- 25 2 500 300 100
28 Y.sub.2 O.sub.3
4 MgO 5 1800 2 4 1 -- 25 2.5 550 300 100
29 Y.sub.2 O.sub.3
4 MgO 10 1800 2 4 1 -- 20 2.5 450 270 100
30 Y.sub.2 O.sub.3
4 MgO 0.5 1400 2 4 55 0.8
1 0.5 40 20 30
31 Y.sub.2 O.sub.3
4 MgO 0.5 1700 2 4 45 1.5
15 1.6 130 80 100
32 Y.sub.2 O.sub.3
4 MgO 0.5 1800 2 4 42 1.8
20 1.8 140 100 100
33 Y.sub.2 O.sub.3
4 MgO 0.5 1900 2 10 35 2.3
25 2 120 80 100
34 Y.sub.2 O.sub.3
4 MgO 0.5 2000 2 100 35 3 30 2.5 70 40 100
35 Y.sub.2 O.sub.3
4 TiO.sub.2
0.5 1800 2 4 45 0.6
12 1.0 150 120 100
36 Y.sub.2 O.sub.3
4 TiO.sub.2
1.2 1800 2 4 42 0.6
10 0.7 200 150 100
37 Y.sub.2 O.sub.3
4 TiO.sub.2
2 1800 2 4 40 0.5
8 0.5 225 170 100
38 Y.sub.2 O.sub.3
4 TiO.sub.2
5 1800 2 4 35 0.5
8 0.5 315 180 100
39 Y.sub.2 O.sub.3
4 TiO.sub.2
10 1800 2 4 28 0.2
4 0.3 421 350 100
40 Y.sub.2 O.sub.3
4 TiO.sub.2
0.5 1600 2 4 52 0.3
7 0.5 72 38 90
41 Y.sub.2 O.sub.3
4 TiO.sub.2
0.5 1700 2 4 50 0.7
8 0.8 180 110 100
42 Y.sub.2 O.sub.3
8 Al.sub.2 O.sub.3
3.5 1650 10 10 18 0.03
0.09
0.04
79 42 72
43 Y.sub.2 O.sub.3
8 Al.sub.2 O.sub.3
0.5 2100 20 100 25 12.5
45 13 62 48 100
44 Y.sub.2 O.sub.3
8 MgO 4.5 1600 10 10 10 0.01
0.02
0.01
66 18 68
45 Y.sub.2 O.sub.3
8 MgO 0.2 2100 15 100 27 15.0
38 5 55 35 100
46 Y.sub.2 O.sub.3
8 TiO.sub.2
4.5 1700 10 10 5 0.04
0.08
0.03
85 41 75
47 Y.sub.2 O.sub.3
8 TiO.sub.2
0.8 2100 20 100 28 12.8
29 8 72 40 100
__________________________________________________________________________
EXAMPLE 4
Silicon oxide powder (20.6 volume %) and yttrium oxide powder (1.2 volume
%) of 0.5 .mu.m mean grain size were added to aluminum nitride powder of
0.5 .mu.m mean grain size, and mixed with an ethanol solvent in a ball
mill for 72 hours.
Mixed powder thus obtained was dried and thereafter compacted using a metal
die of 10 mm.times.10 mm dimensions under a pressure of 20 kg/cm.sup.2
with addition of a compacting assistant. Density of the compact as
obtained was 37% in relative density.
This compact was heat treated in the atmosphere at a temperature of
600.degree. C. for 1 hour for removing the compacting assistant, and
thereafter heat treated in nitrogen at atmospheric pressure at a
temperature of 1700.degree. C. for 1 hour, to obtain a porous body.
Porosity, a mean pore size and a mean aspect ratio of crystal grains of
this porous body were 35%, 1.6 .mu.m and 4 respectively. Three-point
bending strength values at an ordinary room temperature and at
1000.degree. C. were 90 MPa and 60 MPa respectively.
EXAMPLE 5
.alpha.-silicon nitride raw powder materials of 0.3 .mu.m, 7.0 .mu.m and
12.0 .mu.m mean grain size were employed to prepare mixed powder materials
so that yttrium oxide powder contents were 4 volume % in the case of the
powder of 0.3 .mu.m and 5 volume % in the cases of 7.0 .mu.m and 12.0
.mu.m by a method similar to that in Example 1, thereby preparing compacts
having relative density values shown in Table 4. Compact density values
were adjusted by changing uniaxial compacting pressures in the range of at
least 1 kg/cm.sup.2 and not more than 2000 kg/cm.sup.2. The compacts as
obtained were treated and evaluated under the same conditions as those in
Example 1 except that heat treatments after decomposition of a compacting
assistant were carried out under the same conditions in nitrogen of 4 atm.
at a temperature of 1800.degree. C. for 2 hours. The evaluation results
are shown in Table 4.
From these results, it is understood that it is possible to control the
mean pore sizes of the porous bodies obtained after the heat treatments by
controlling the mean grain sizes of the raw powder materials and the
density values of the compacts.
TABLE 4
__________________________________________________________________________
Characteristics
Crystal
Raw Compact
Heat Treatment Conditions
Mean
Grain Size
Bending Strength
.beta.-
Material
Additive
Relative Retention
Pressure of
Pore
Major
Minor
Room Transition
Grain Size
Y.sub.2 O.sub.3
Density
Temperature
Time Atmosphere
Porosity
Size
Axis
Axis
Temperature
1000.degree.
Ratio
(.mu.m)
(Vol. %)
(%) (.degree.C.)
(H) (atm) (%) (.mu.m)
(.mu.m)
(.mu.m)
(MPa) (MPa)
(%)
__________________________________________________________________________
0.3 4 20 1800 2 4 72 1.6
22 1.7 40 35 100
0.3 4 25 1800 2 4 70 1.6
20 1.7 60 50 100
0.3 4 27 1800 2 4 67 1.5
20 1.5 70 50 100
0.3 4 30 1800 2 4 60 1.2
18 1.2 100 80 100
0.3 4 35 1800 2 4 48 0.8
15 1.0 120 100 100
0.3 4 40 1800 2 4 42 0.6
10 0.8 150 130 100
0.3 4 45 1800 2 4 40 0.5
6 0.5 180 150 100
0.3 4 50 1800 2 4 38 0.2
4 0.2 210 180 100
0.3 4 55 1800 2 4 35 0.1
2 0.1 280 230 100
0.3 4 60 1800 2 4 31 0.05
1 0.07
350 280 100
0.3 4 65 1800 2 4 27 0.03
1 0.06
400 350 100
0.3 4 70 1800 2 4 20 0.02
1 0.05
450 400 100
7.0 5 20 1800 2 4 50 5.1
22 2.5 50 40 100
7.0 5 28 1800 2 4 47 3.8
20 2.1 60 50 100
7.0 5 30 1800 2 4 43 2.4
18 1.7 88 70 100
7.0 5 40 1800 2 4 40 1.8
15 1.2 130 100 100
7.0 5 50 1800 2 4 38 1.2
14 1.1 210 150 100
7.0 5 60 1800 2 4 32 0.7
12 0.8 220 180 100
7.0 5 65 1800 2 4 19 0.3
10 0.5 250 200 100
12.0 5 20 1800 2 4 60 6 28 3.0 50 30 100
12.0 5 28 1800 2 4 60 4 25 2.5 82 65 100
12.0 5 30 1800 2 4 58 3.5
16 1.8 105 88 100
12.0 5 40 1800 2 4 53 3.1
12 1.7 170 103 100
12.0 5 50 1800 2 4 50 2.0
8 1.4 190 120 100
12.0 5 60 1800 2 4 37 1.5
7 1.3 210 180 100
12.0 5 65 1800 2 4 28 1.2
5 1.1 240 200 100
12.0 5 20 2100 2 100 25 13.2
45 11 43 18 100
__________________________________________________________________________
EXAMPLE 6
Silicon nitride ceramics porous bodies of 0.1 to 5.0 .mu.m in mean pore
size which were prepared by the inventive preparation method were worked
into the form of discs of .phi.25 mm.times.0.5 mm thickness. These porous
bodies were employed to carry out permeation experiments through isopropyl
alcohol (20.degree. C.) and pure water (20.degree. C.). The results are
shown in Table 5. Table 5 shows flow rate results in a case of employing
.alpha.-alumina ceramics porous bodies having the same pore sizes as
comparative examples.
It is understood from the results that the silicon nitride porous bodies
have higher performance with regard to liquid permeation than flow rates
the alumina porous bodies.
TABLE 5
______________________________________
Grain
Size Porosity
IPA Flow Rate
Pure Water Flow Rate
Material (.mu.m)
(%) (ml(min/cm.sup.2)
(ml/min/cm.sup.2)
______________________________________
Silicon Nitride
0.1 45 0.82 1.97
Silicon Nitride
0.2 48 2.01 4.82
Silicon Nitride
0.5 60 4.11 9.86
Sliicon Nitride
1.0 60 14.1 33.8
Silicon Nitride
2.0 55 22.5 54.0
Silicon Nitride
5.0 50 40.3 96.7
.alpha.-Alumina
0.1 40 0.43 1.02
.alpha.-Alumina
0.2 40 1.06 2.55
.alpha.-Alumina
0.5 40 1.78 4.25
.alpha.-Alumina
1.0 40 4.96 11.9
.alpha.-Alumina
2.0 40 8.85 21.25
.alpha.-Alumina
5.0 40 17.7 42.5
______________________________________
IPA (isopropyl alcohol) flow rates and pure water flow rates are
permeation flow rates in pressurization at 20.degree. C. and 1.0
kg/cm.sup.2.
According to the present invention, as hereinabove described, it is
possible to obtain a ceramics porous body having high porosity and high
strength. This porous body, which is excellent in high temperature
characteristics and chemical resistance, is useful as a filter which is
employed at a high temperature or a catalytic carrier which is employed in
an atmosphere having high corrosiveness.
Although the invention has been described with reference to specific
example embodiments, it will be appreciated that it is intended to cover
all modifications and equivalents within the scope of the appended claims.
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